Add light/dark theme toggle

CSS custom properties overridden via [data-theme="dark"] with
prefers-color-scheme fallback. Toggle in nav persists choice to
localStorage. No Go changes — CSS and template only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/static/style.css

@@ -16,6 +16,46 @@
   --code-bg: #f0ede4;
   --green:   #4a7c40;
   --blue:    #3b5998;
+  --error-bg: #fdf0f0;
+  --error-border: #e8c4c4;
+}
+
+/* Dark theme — explicit toggle */
+[data-theme="dark"] {
+  --bg:      #1a1a1a;
+  --bg-alt:  #242420;
+  --text:    #d4d0c8;
+  --text-dim: #999;
+  --text-lt: #666;
+  --accent:  #d44040;
+  --accent-h: #e85050;
+  --border:  #3a3a36;
+  --border-lt: #2e2e2a;
+  --code-bg: #2a2a24;
+  --green:   #6aac58;
+  --blue:    #5b89c8;
+  --error-bg: #2a1a1a;
+  --error-border: #4a2a2a;
+}
+
+/* Dark theme — system preference fallback */
+@media (prefers-color-scheme: dark) {
+  html:not([data-theme]) {
+    --bg:      #1a1a1a;
+    --bg-alt:  #242420;
+    --text:    #d4d0c8;
+    --text-dim: #999;
+    --text-lt: #666;
+    --accent:  #d44040;
+    --accent-h: #e85050;
+    --border:  #3a3a36;
+    --border-lt: #2e2e2a;
+    --code-bg: #2a2a24;
+    --green:   #6aac58;
+    --blue:    #5b89c8;
+    --error-bg: #2a1a1a;
+    --error-border: #4a2a2a;
+  }
 }
 
 /* ===========================
@@ -82,6 +122,20 @@ code {
   font-size: 0.8125rem;
   color: var(--text-dim);
 }
+.theme-toggle {
+  background: none;
+  border: none;
+  cursor: pointer;
+  font-size: 1rem;
+  padding: 0.25rem;
+  line-height: 1;
+  color: var(--text-dim);
+}
+.theme-toggle:hover {
+  background: none;
+  border: none;
+  color: var(--text);
+}
 
 /* ===========================
    Page containers
@@ -147,9 +201,9 @@ code {
    Alerts
    =========================== */
 .error {
-  background: #fdf0f0;
+  background: var(--error-bg);
   color: var(--accent);
-  border: 1px solid #e8c4c4;
+  border: 1px solid var(--error-border);
   padding: 0.625rem 1rem;
   border-radius: 3px;
   margin-bottom: 1rem;